1. Field of the Invention
The present invention relates to the manufacturing of vertical unipolar components in monolithic form. The following description more specifically aims, as an example only, at the case of components of Schottky diode type made in vertical form in silicon substrates.
2. Discussion of the Related Art
FIG. 1 illustrates a conventional structure of a Schottky diode. The structure comprises a heavily-doped semiconductor substrate (N+) 1, typically made of single-crystal silicon. A cathode layer (N−) 2 more lightly doped than substrate 1 covers substrate 1. A metal layer 3 forms a Schottky contact with cathode 2 and forms the diode anode. The contour of anode 3 is defined by an insulator 4.
FIG. 1 also illustrates, in dotted lines, a distribution of equipotential surfaces V1, V2, V3, V4, and V5 in cathode 2 upon reverse biasing at a voltage V5. Equipotential surfaces V1 to V5 are equidistant, the intermediary equipotential surfaces V1 to V4 corresponding to increasing fractions of voltage V5 (V1=V5/5, V2=2V5/5, V3=3V5/5, V4=4V5/5). Equipotential surfaces V1 to V5 distribute in homogeneous fashion in cathode 2 and exhibit an area of strong curvature at the periphery of Schottky junction 2-3.
The forming of such unipolar components comes up against two opposite constraints. They must exhibit the smallest possible on-state resistance (Ron) while having a high breakdown voltage.
Minimizing the on-state resistance requires minimizing the thickness of the less heavily-doped layer (layer 2) and maximizing the doping of this layer.
On the contrary, to obtain a high reverse breakdown voltage, the doping of layer 2 must be minimized and its thickness must be maximized, while avoiding creating areas in which the equipotential surfaces are strongly curved.
Various solutions have been provided to reconcile such opposite constraints. The structure and operation of two of these solutions will be briefly described hereafter in relation with FIGS. 2 and 3.
In the structure of FIG. 2, vertical doped regions (P) 20 are formed in a cathode layer (N) 21 more lightly doped than an underlying substrate (N+) 22. Regions 20 extend across the thickness of cathode 21 from its upper surface. A metal layer 23 covers the entire structure, forming a Schottky contact with cathode 21 and also contacting the upper surface of regions 20. The dimensions and the doping of regions 20 are selected so that the amount of dopants of all regions 20 is equal to the amount of dopants of opposite type present in the portions of the part of cathode 21 separating regions 20. If the width of regions 20 is equal to the interval separating them, their doping is equal to the doping of layer 21.
In reverse biasing, the portions of layer 21 separating two regions 20 progressively deplete from Schottky interface 21-23 and from P-N interface 20-21. When these portions are completely depleted, the assembly of cathode 21 and of regions 20 substantially behaves as an almost intrinsic layer of zero doping. FIG. 2 illustrates in dotted lines the distribution of equidistant equipotential surfaces V11, V12, V13, V14, and V15 upon reverse biasing at a value V15. Equipotential surfaces V11 to V15 are homogeneously distributed and are substantially planar. Thus, the doping of layer 21 may be increased while providing an optimal breakdown voltage.
However, such a structure is complex to manufacture. Indeed, to ensure the necessary control of the dimensions and doping of regions 20, said regions are formed by implantation upon epitaxial growth of cathode layer 21. To form deep vertical regions 20 with a substantially homogeneous doping, masking, implantation, and epitaxy steps must be repeated.
In the structure of FIG. 3, conductive areas (N+) 30, for example made of heavily-doped N-type polysilicon, are formed in an upper portion of a thick layer (N) 31 less heavily N-type doped than an underlying substrate (N+) 32. An insulating layer 33 insulates areas 30 from layer 31. A metal anode 34 covers the entire structure, contacting the upper surface of areas 30 and forming a Schottky contact with cathode 31.
In reverse biasing, insulated areas 30 cause a lateral depletion of layer 31, which modifies the equipotential surface distribution in layer 31 with respect to the distribution in homologous layer 2 of FIG. 1. This enables increasing the doping of layer 31 with respect to the doping of layer 2, and thus reducing the on-state resistance with no adverse effect on the reverse breakdown voltage.
FIG. 3 illustrates in dotted lines the distribution of equidistant equipotential surfaces V21 to V25 upon reverse biasing at a value V25. Equipotential surfaces V21 to V25 are homogeneously distributed in cathode 31, but avoid areas 30 by passing, partly, through insulating layer 33. The equipotential surfaces corresponding to the highest voltages then exhibit significant curvatures at the level of the angles of layer 33, and a breakdown will occur first at these locations.
For a given on-state resistance, the reverse breakdown voltage will thus be smaller with the structure of FIG. 3 than with the structure of FIG. 2. In practice, despite their limited performances, it is however preferred to use such structures, since they are easier to manufacture than those of FIG. 2. It is indeed possible to form by epitaxy cathode 31 in a single step, then dig trenches, coat them with insulating layer 33, and fill them with the conductive material.